1. Field of the Invention
The present invention relates to a sheet conveying apparatus, and an image reading apparatus and image forming apparatus having the sheet conveying apparatus and, more particularly, to a structure for correcting skew feed of a sheet.
2. Related Background Art
Some conventional image reading apparatuses or image forming apparatuses have a sheet feeding apparatus which corrects skew feed of a sheet by pressing the sheet conveyed by a conveying means against a registration roller pair to form a loop in the sheet and then conveys the sheet by rotating the registration rollers.
FIG. 10 is a view showing the structure of such a conventional sheet feeding apparatus. Referring to FIG. 10, a sheet stocking tray 60 stocks a sheet of paper (not shown). A pickup roller 61 picks up the sheet. A shutter 62 restrains the leading ends of the sheets set on the sheet stocking tray 60 and is released at a predetermined timing. A weight 63 is arranged at a position opposite to the pickup roller 61 and, in feeding the sheet, moves downward and clamps (a batch of) sheets to apply a sheet feeding force to them.
A sheet presence/absence sensor lever 64 is arranged upstream of the shutter 62 to detect the presence/absence of a sheet. When a sheet is abutted against the shutter 62 and set, the sheet presence/absence sensor lever 64 is pressed by the sheet and pivots to shield light to a photointerrupter (not shown). When the light to the photointerrupter is shielded, a signal representing that the light is shielded is input to a control section (not shown). On the basis of the signal, the control section detects that the sheet is set.
A lower guide plate 65 guides the sheet. An upper guide plate 66 guides the sheet. A feed roller 67 feeds the sheet. A separation belt section 68 separates the stocked sheets one by one. A loop formation space 69 allows the sheet to loop to correct skew feed of the sheet. A sheet lower guide plate 70 defines the lower surface of the loop formation space 69. A sheet upper guide plate 75 defines the upper surface of the loop formation space 69.
An ante-registration sensor lever 71 detects the leading end of the sheet separated one by one and conveyed by the feed roller 67 and the separation belt section 68. The ante-registration sensor lever 71 is pressed by the conveyed sheet and pivots to shield light to a photointerrupter (not shown). When the light to the photointerrupter is shielded, a signal representing that the light is shielded is input to a control section. On the basis of the signal, the control section detects that the sheet is conveyed.
A registration roller pair 72, 73 temporarily stop the leading end of the sheet separated one by one and conveyed by the feed roller 67 and the separation belt section 68 and forms a loop in the sheet, thereby correcting skew feed of the sheet. A sheet leading end detection lever 74 detects the leading end of the sheet conveyed by the registration roller pair 72, 73 after the skew feed of the sheet is corrected. The sheet leading end detection lever 74 has the same structure as that of the above-described sheet presence/absence sensor lever 64 or the ante-registration sensor lever 71.
The feed operation of the sheet feeding apparatus having the above arrangement will be described next.
When a user stocks sheets on the sheet stocking tray 60 and sets them by abutting the leading ends of the sheets against the shutter 62, the sheet presence/absence sensor lever 64 is pressed by the sheets and pivots, and the control section detects that the sheets are set. Upon detecting that the sheets are set, the control section makes the shutter 62 pivot downward and the weight 63 move downward and clamp the sheets with the pickup roller 61.
Next, the pickup roller 61 is rotated at a predetermined timing. Each batch of sheets is guided by the lower guide plate 65 and the upper guide plate 66 and conveyed to a feed separation section constituted by the feed roller 67 and the separation belt section 68. After that, the conveyed batch of sheets is separated one by one and conveyed to the registration roller pair 72, 73 by the feed roller 67.
Each conveyed sheet reaches the registration roller pair 72, 73 while making the ante-registration sensor lever 71 pivot. At this time, the registration roller pair 72, 73 are kept in a stop state. The conveyed sheet abuts against the registration roller pair 72, 73, and the leading end of the sheet stops.
Even when the leading end of the sheet stops, the feed roller 67 continuously feeds the sheet for a predetermined time from a time when the leading end of the sheet is detected by the ante-registration sensor lever 71. Hence, a loop is formed in the loop formation space 69 between the registration roller pair 72, 73 and the feed roller 67 to correct skew feed of the sheet.
After the skew feed of the sheet is corrected by forming the loop, the control section rotates the registration roller pair 72, 73. At this time, since the feed roller 67 is kept stopped, the sheet is conveyed only by the registration roller pair 72, 73. As a result, the sheet is conveyed to the next process by the registration roller pair 72, 73 while attenuating the loop.
In such a conventional sheet feeding apparatus, however, a loop is formed in a different manner depending on the type of sheet. For a certain type of sheet, a loop is abruptly formed, and the sheet strongly abuts against the sheet upper guide plate 75 in the loop formation space 69 to generate large noise. If such large noise is generated every time a sheet is fed, the noise level of the apparatus becomes considerably high.
When the registration roller pair 72, 73 rotate after loop formation, the loop of sheet disappears. If the loop abruptly disappears, the sheet strongly abuts against the sheet lower guide plate 70, resulting in larger noise.
A cardboard, a thick sheet of paper, or a strongly rigid sheet remains straight without forming a loop because of its rigidity even when the sheet abuts against the registration roller pair 72, 73 that stand still, and no sufficient loop can be formed. In this case, skew feed of the sheet cannot be sufficiently corrected.
It is an object of the present invention to provide a sheet feeding apparatus capable of reducing noise and reliably correcting skew feed of a sheet, and an image reading apparatus and an image forming apparatus having the sheet feeding apparatus.
According to the present invention, there is provided a sheet conveying apparatus which corrects skew feed of a sheet by pressing the sheet conveyed by conveying means against a stationary registration roller pair so as to form a loop in the sheet and then rotates the registration roller pair to convey the sheet, comprising a loop formation space provided between the conveying means and the registration roller pair, in which the sheet forms the loop, an upper member for defining an upper surface of the loop formation space, the upper member extending in a direction of the registration roller pair which is to abut against the sheet when the sheet forms the loop, and a lower member for defining a lower surface of the loop formation space, wherein the lower member is designed to move the sheet conveyed by the conveying means toward the registration roller pair along the upper member and move downward when pressed by the sheet conveyed by the registration roller pair while attenuating the loop.
According to the present invention, preferably, the conveying means comprises a conveying member and separation means for separating the sheet upon abutting against the conveying member, and the lower member has an apex which projects upwardly with respect to a line that connects a nip of the registration roller pair and a nip of the conveying member and the separation means.
According to the present invention, the apparatus may preferably include one or a plurality of runners rotatably arranged at the apex of the lower member.
According to the present invention, the apparatus may preferably include an elastic member for biasing the lower member upward.
According to the present invention, there is also provided a sheet conveying apparatus comprising registering means for correcting skew feed of a sheet by restraining a leading end of the sheet conveyed by conveying means so as to form a loop in the sheet and then conveying the sheet, a first guide for guiding the sheet to the registering means, the first guide abutting against a projecting surface of the sheet which forms the loop, and a second guide for guiding the sheet and causing the sheet to abut against the first guide before abutting against the registering means, wherein the second guide moves when pressed by the sheet conveyed by the registering means while attenuating the loop.
According to the present invention, an image reading apparatus may preferably include any one of the above sheet conveying apparatuses and an image reading section for reading an image formed on a sheet conveyed by the sheet conveying apparatus.
According to the present invention, an image forming apparatus may include any one of the above sheet conveying apparatuses and an image forming section for forming an image on a sheet conveyed by the sheet conveying apparatus.
As in the present invention, a sheet conveyed by a conveying means is pressed against a stationary registration roller pair, thereby forming a loop in the sheet in a loop formation space between the conveying means and the registration roller pair. A lower member which defines the lower surface of the loop formation space has a shape with which the sheet conveyed by the conveying member moves toward the registration roller pair along an upper member which defines the upper surface of the loop formation space in forming the loop. When the lower member is pressed by the sheet conveyed by the registration roller pair while attenuating the loop, the lower member moves downward.